1. Field of the Invention
The present invention relates to a chargeable battery device, and more particularly, to a battery state monitoring circuit for monitoring charging and discharging of a battery device.
2. Description of the Related Arts
FIG. 3 is a block diagram showing a conventional battery device. In the conventional battery device, a negative electrode of a secondary battery 301 is connected with a negative external terminal 313 with which one end of a charger 310 or one end of an external load 311 is to be connected. A positive electrode of the secondary battery 301 is connected with a positive external terminal 312 with which the other end of the charger 310 or the other end of the external load 311 is to be connected, through a switch circuit 302 and a switch circuit 303 which are connected in series. The secondary battery 301 is connected in parallel with a battery state monitoring circuit 316. The battery state monitoring circuit 316 includes a voltage detecting circuit 317 for detecting a voltage of the secondary battery 301 and an over-current detecting circuit 319 for detecting a voltage generated between an over-current detection terminal 309 and a VDD terminal 314.
The battery state monitoring circuit 316 has a function of turning OFF the switch circuit 303 when a state in which a battery voltage of the secondary battery 301 is higher than a predetermined voltage value is detected. This state is referred to as an “over-charge detection state”. In addition, the battery state monitoring circuit 316 has a function of turning OFF the switch circuit 302 when a state in which the battery voltage of the secondary battery 301 is lower than a predetermined voltage value is detected. This state is referred to as an “over-discharge detection state”. Further, the battery state monitoring circuit 316 has a function of turning OFF the switch circuit 302 when a state in which an inter-terminal voltage generated between the over-current detection terminal 309 and the VDD terminal 314 becomes larger than a predetermined voltage value by an increase in current flowing through the switch circuits 302 and 303, each of which has a predetermined ON resistance. This state is referred to as an “over-current detection state”. In the over-current detection state, a PMOSFET 306 is being turned ON, so the over-current detection terminal 309 is pulled up to the VDD terminal 314 through a resistor circuit 307. The pull-up is used to release the over-current detection state. That is, when a resistance value of the external load 311 is sufficiently larger than that of the resistor circuit 307, a voltage at the over-current detection terminal 309 becomes close to a voltage at the VDD terminal 314. Then, the inter-terminal voltage generated between the over-current detection terminal 309 and the VDD terminal 314 becomes smaller than the predetermined voltage value. The over-current detecting circuit 319 detects the inter-terminal voltage and generates a release signal.
However, according to the conventional battery device, the switch circuit 303 is being turned OFF in the over-charge detection state. Therefore, the voltage at the over-current detection terminal 309 is increased to a value larger than the voltage at the VDD terminal 314 by the charger 310, so a leakage current 320 flows from the over-current detection terminal 309 to the VDD terminal 314 through the resistor circuit 307 and a parasitic capacitor 308. The leakage current 320 flows from the VDD terminal 314 into the secondary battery 301. Thus, there is a problem in that it is likely to continue charging even in the over-charge detection state.
In the conventional battery state monitoring circuit and the conventional battery device, the resistance value of the resistor circuit 307 is increased to limit the leakage current 320 to a small value. Therefore, even when the resistance value of the resistor circuit 307 is to be reduced in view of the release of the over-current detection state, there is a problem in that the reduction in resistance value cannot be realized because the leakage current 320 becomes larger.